Refactor code to remove duplicate error handling.

Amends: r2580

git-svn-id: svn://daper.net/moc/trunk@2584 910807d9-36e0-0310-a014-e9ea483e2ba4
Fix handling of unknown size result from io_file_size().

git-svn-id: svn://daper.net/moc/trunk@2585 910807d9-36e0-0310-a014-e9ea483e2ba4
Warn that MOC 2.6 will require Autoconf version 2.64 or later.

git-svn-id: svn://daper.net/moc/trunk@2586 910807d9-36e0-0310-a014-e9ea483e2ba4